This invention relates to optoelectronic devices and, more particularly, to organic light emitting devices (organic EL devices). More specifically, the present invention relates to substantially stable organic EL devices and devices possessing extended operational life time, such as at least about 1,000 hours in embodiments, which is prior to their luminance decreasing to some percent of its initial luminance value, such as about 50 percent of the initial luminance, and which devices do not in embodiments, for example, usually degrade in the form of experiencing a decrease in their luminance at high temperatures, such as about 100xc2x0 C., and moreover, which devices are not substantially adversely affected by high temperatures, such as having a life time of at least about 100 hours at these temperatures.
The organic light emitting devices of the present invention can be utilized in various devices, such as displays that typically are operated over a broad range of temperature conditions. The operational stability at high temperature conditions provided by the organic light emitting devices of this invention enables in embodiments the use of these devices at high temperatures for extended periods of time.
An organic electroluminescent (EL) device can be comprised of a layer of an organic luminescent material interposed between an anode, typically comprised of a transparent conductor, such as indium tin oxide, and a cathode, typically a low work function metal such as magnesium, calcium, aluminum, or the alloys thereof with other metals. The EL device functions on the primary principle that under an electric field, positive charges (holes) and negative charges (electrons) are respectively injected from the anode and cathode into the luminescent layer and undergo recombination to form excitonic states which subsequently emit light. A number of organic EL devices have been prepared from a laminate of an organic luminescent material and electrodes of opposite polarity, which devices include a single crystal material, such as single crystal anthracene as the luminescent substance as described, for example, in U.S. Pat. No. 3,530,325, the disclosure of which is totally incorporated herein by reference. These types of devices are believed to require excitation voltages on the order of 100 volts or greater.
An organic EL device with a multilayer structure can be formed as a dual layer structure comprising one organic layer adjacent to the anode supporting hole transport, and another organic layer adjacent to the cathode supporting electron transport and acting as the organic luminescent zone of the device. Examples of these devices are disclosed in U.S. Pat. Nos. 4,356,429; 4,539,507; 4,720,432, and 4,769,292, the disclosures of which are totally incorporated herein by reference, wherein U.S. Pat. No. 4,769,292, the disclosure of which is totally incorporated herein by reference, discloses, for example, an organic EL device comprising three separate layers, a hole transport layer, a luminescent layer, and an electron transport layer, which layers are laminated in sequence and are sandwiched between an anode and a cathode, and wherein a fluorescent dopant material is added to the emission zone or layer whereby the recombination of charges results in the excitation of the fluorescent material. In some of these multilayer structures, such as, for example, organic light emitting devices described in U.S. Pat. No. 4,720,432, the disclosure of which is totally incorporated herein by reference, the organic light emitting device further comprises a buffer layer interposed between the hole transport layer and the anode. The combination of the hole transport layer and the buffer layer forms a dual-layer hole transport region, reference S. A. Van Slyke et al., xe2x80x9cOrganic Electroluminescent Devices with Improved Stability,xe2x80x9d Appl. Phys. Lett. 69, pp. 2160-2162, 1996, the disclosure of which is totally incorporated herein by reference.
There have also been attempts to obtain electroluminescence from organic light emitting devices containing mixed layers, for example, layers in which both the hole transport material and the emitting electron transport material are mixed together in one single layer, see, for example, Kido et al., xe2x80x9cOrganic Electroluminescent Devices Based On Molecularly Doped Polymers,xe2x80x9d Appl. Phys. Lett. 61, pp. 761-763, 1992; S. Naka et al., xe2x80x9cOrganic Electroluminescent Devices Using a Mixed Single Layer,xe2x80x9d Jpn. J. Appl. Phys. 33, pp. L1772-L1774, 1994; W. Wen et al., Appl. Phys. Lett. 71, 1302 (1997); and C. Wu et al., xe2x80x9cEfficient Organic Electroluminescent Devices Using Single-Layer Doped Polymer Thin Films with Bipolar Carrier Transport Abilitiesxe2x80x9d, IEEE Transactions on Electron Devices 44, pp. 1269-1281, 1997. In a number of these devices, the electron transport material and the emitting material can be the same or the mixed layer can further comprise an emitting material as a dopant. Other examples of organic light emitting devices which are formed of a single organic layer comprising a hole transport material and an electron transport material can be found, for example, in U.S. Pat. Nos. 5,853,905; 5,925,980; 6,114,055 and 6,130,001, the disclosures of which are totally incorporated herein by reference. As indicated in the article by S. Naka et al., these single mixed layer organic light emitting devices are generally less efficient than multilayer organic light emitting devices. These devices, which include only a single mixed layer of a hole transport material, such as NBP (N,Nxe2x80x2-di(naphthalene-1-yl)-N,Nxe2x80x2-diphenyl-benzidine), and an emitting electron transport material, such as Alq3 (tris (8-hydroxyquinoline) aluminum), are believed to be unstable and to have poor efficiency. The instability of these devices is believed to be caused by the direct contact between the electron transport material in the mixed layer and the hole injecting contact comprised of indium tin oxide (ITO), which results in the formation of an unstable cationic electronic transport material, and the instability of the mixed layer/cathode interface, see H. Aziz et al., Science 283, 1900 (1999), the disclosure of which is totally incorporated herein by reference. In addition, the single mixed layer may result in high leakage currents and hence poor efficiency, see Z. D. Popovic et al., Proceedings of the SPIE, Vol. 3176, xe2x80x9cOrganic Light-Emitting Materials and Devices IIxe2x80x9d, San Diego, Calif., Jul. 21-23, 1998, pp. 68 to 73, the disclosure of which is totally incorporated herein by reference.
While recent progress in organic EL research has elevated the potential of organic EL devices for widespread applications, the operational stability of current available devices may in some instances be below expectations. A number of known organic light emitting devices have relatively short operational lifetimes before their luminance drops to some percentage of its initial value. Providing interface layers as described, for example, in S. A. Van Slyke et al., xe2x80x9cOrganic Electroluminescent Devices with Improved Stability,xe2x80x9d Appl. Phys. Lett. 69, pp. 2160-2162, 1996, and doping as described, for example, in Y. Hamada et al., xe2x80x9cInfluence of the Emission Site on the Running Durability of Organic Electroluminescent Devicesxe2x80x9d, Jpn. J. Appl. Phys. 34, pp. L824-L826, 1995, may perhaps increase the operational lifetime of organic light emitting devices for room temperature operation, however, the effectiveness of these organic light emitting devices deteriorates for high temperature device operation. In general, the device lifetime can be reduced by a factor of about two for each 10xc2x0 C. increment in the operational temperature. Moreover, at high temperatures, the susceptibility of the organic light emitting devices to degrade is increased as described, for example, in Zhou et al., xe2x80x9cReal-Time Observation of Temperature Rise and Thermal Breakdown Processes in Organic Leds Using an IR Imaging And Analysis Systemxe2x80x9d, Advanced Materials 12, pp 265-269, 2000, which further reduces the stability of the devices. As a result, the operational lifetime of these organic light emitting devices at a normal display luminance level of about 100 cd/m2 is limited, for example, to about a hundred hours or less at temperatures of about 60xc2x0 C. to about 80xc2x0 C., reference J. R. Sheats et al., xe2x80x9cOrganic Electroluminescent Devicesxe2x80x9d, Science 273, pp. 884-888, 1996, and also S. Tokito et al., xe2x80x9cHigh-Temperature Operation of an Electroluminescent Device Fabricated Using a Novel Triphenylamine Derivativexe2x80x9d, Appl. Phys. Lett. 69, 878 (1996).
This invention provides in embodiments organic light emitting devices with, in embodiments thereof, enhanced operational lifetimes. The organic light emitting devices according to embodiments of this invention can provide operational stability at high temperatures, such as, for example, an operational lifetime of several hundreds of hours, such as 1,200 hours at a high brightness of, for example, about 1,500 candelas per square meter (cd/m2) at temperatures of from about 80xc2x0 C. to about 100xc2x0 C., which corresponds to, for example, about 10,000 hours for a typical display luminance of about 100 cd/m2 at temperatures of from about 80xc2x0 C. to about 100xc2x0 C.
The organic light emitting devices according to the embodiments of the present invention comprise
(i) a first electrode;
(ii) a mixed region comprising a first hole transport material, which material can also function as a luminescent material, and a first electron transport material, which material can also function as a luminescent material, and which mixed region can also further include an organic luminescent material, and wherein the mixed region is capable of emitting light in response to hole electron recombination;
(iii) a second electrode;
(iv) an optional thermal protective element coated usually present when the device is operated at higher temperatures, for example about equal to or about above 70xc2x0 C. to about 100xc2x0 C., and which element needs not be present when the device is operating at lower temperatures, such as for example, from about 20xc2x0 C. to about 70xc2x0 C. on one of the first and second electrodes, wherein one of the first and second electrodes is a hole injection anode, and one of the electrodes is an electron injection cathode, and wherein the organic light emitting device further comprises at least one of
(v) a hole transport region, interposed or situated between the anode and the mixed region; and
(vi) an electron transport region interposed between the cathode and the mixed region, and wherein
a. the hole transport region, which region can be one or more layers, contains a second hole transport material at least in part, or in one layer where the hole transport region is in contact with the mixed region, and wherein the first hole transport material (ii), and the second hole transport material (vi) are not the same material or dissimilar; and
b. the electron transport region, similar to a. above, comprises a second electron transport material at least in a part, reference a. above, where the electron transport region is bordering or is in contact with the mixed region, and wherein the first electron transport material and the second electron transport material are not the same material, or are dissimilar. For example, when the hole transport region contains two layers, the layer in contact with the mixed region can be dissimilar than the hole transport material of the mixed region.
The use of different electron materials in the mixed region (ii) and in the electron transport region (vi), or at least in the layer of the electron transport region where the region is bordering the mixed region, and/or the use of different hole transport materials in the mixed region (ii), the hole transport region (v), or at least in the layer of the hole transport region (v) b. ordering the mixed region can provide the organic light emitting devices with a variety of desirable features, such as, for example, (1) excellent and in embodiments increased efficiency and/or stability, (2) simple and economic fabrication, and/or (3) greater latitude in device design and materials selection. For example, the use of different electron transport materials in the mixed region and in the electron transport region, or at least in part of the electron transport region, where the electron transport region is bordering the mixed region, can be effective in increasing the efficiency of organic light emitting device by creating an energy barrier at the interface between the mixed region and the electron transport region, resulting in a substantial decrease in energy losses through exciton diffusion and subsequent quenching by the electrodes or through leakage of holes to the cathode. Similarly, the use of different hole transport materials in the mixed region and the hole transport region, or at least in the part where the hole transport region is bordering the mixed region can further increase the efficiency of the organic light emitting device. Organic light emitting devices comprising a mixed region of a hole transport material and an electron transport material results in close spatial proximity between molecules of the hole transport material and molecules of the electron transport material in these mixed regions, which can result in intermolecular interactions, and depending on the materials used, can cause undesirable side effects, such as, for example, complex formation or other exciton quenching effects, and hence can be detrimental to the efficiency or the stability of the organic light emitting device comprising such mixed layers. These material compatibility issues may limit the choice of hole transport materials and electron transport materials that can be used in forming these mixed layers. Therefore, an advantage of selecting materials in the hole transport region or the electron transport region that are different from those in the mixed region can permit a greater latitude in the selection of materials that can be used in forming the hole transport region and/or the electron transport region, and permits the use of materials that are simpler to synthesize and hence, potentially more economical.
With further respect to the EL devices of the present invention, the mixed region (ii) can contain a luminescent material or compound; also in embodiments, the hole transport material can further function as a luminescent component; the electron transport material of (ii) can also further function as a luminescent component, and moreover, the mixed region can include therein in the aforementioned embodiments a third luminescent component. In embodiments, there is added to the mixed region a separate luminescent compound.